This invention relates to a web feed apparatus and particularly to web feed apparatus for intermittent moving of a web in a bag machine and the like.
In the formation of successive similar elements for a web structure from a web source, successive lengths of the material may be moved through a forming machine in a time stepped manner. For example, in a bag making machine, a web of indefinitive length is moved by appropriate feed rolls from a supply into a sealing and cutoff machine or apparatus. For example, U.S. Pat. No. 3,722,376 which issued Mar. 27, 1973 and U.S. Pat. No. 4,425,988 which issued Jan. 17, 1984 disclose bag making machines including a seal and cutoff unit with a reciprocating sealing bar. A web of indefinite length is passed from a supply beneath the sealing bar and a platen in the raised or open position, and then stopped. The bar moves down to seal and thermally sever the web and form a bag. The bag length is controlled by the length of web fed into the sealing and severing unit.
Various means for intermittent feeding of the web have been suggested.
The web feed apparatus generally includes feed rolls adapted to pressure engage the web and transport the web into and through the seal and severing bar apparatus. A drive unit provides for acceleration of the web for feeding of the web followed by a corresponding dwell period during which the seal bar unit is operative to seal and sever the web. The drive rolls are generally coupled through a clutch-brake system or mechanism to establish appropriate interrelated relation between the feed rolls and the other components of the bag making machine including the seal bar unit.
A harmonic drive system is used to control the intermittent motion of the indexing feed rolls and web in a bag machine. The drive establishes a sine wave motion to the input of a clutch-brake unit between a drive motor and the feed rolls. The harmonic drive system consists of a variable radius crank shaft which is connected to a gear rack engaged with a pinion gear. The pinion gear indirectly drives the input of the clutch-brake unit. The web movement and bag length is determined by the crank radius and is changed by changing the radius of the crank shaft through use of a bi-directional motor.
The web is moved by energizing the clutch during the positive half cycle of the sine wave. During the negative half cycle of the sine wave, the brake is energized. Sealing, cutting and other auxiliary functions are performed on the web with the feed rolls stationary. The clutch and brake switching must be done at the zero velocity cross-over points in the sine wave to eliminate shock loading of the machine and rapid wear of the clutch brake. The machine is therefore limited to a 180 degree clutch and feed cycle and a 180 degree brakework cycle.
A second system used on bag machines consists of a motor driven positive infinitely variable (PIV) drive driving the input shaft of a clutch-brake system at a constant speed. The clutch-brake system is electrically actuated to set either the clutch or the brake to alternately move the web or hold the web stationary. A cam, essentially a duplicate of the seal unit, actuates the clutch-brake systems, and can accommodate an unequal clutch-brake cycle because the clutch input shaft speed does not change during the machine cycle and engagement speed is always the same. However, the variable drive system is generally limited to reduced cycle rate machines and causes a more rapid wear of the clutch-brake system than the sine wave harmonic drive system. Bag length is changed by changing the output speed of the variable drive with a constant input. Thus, if machine cycle rate remains constant, the feed rolls rotate further during the same time period thus developing a longer bag.